megacrystic granite, granodiorite and charnockite belonging to the main De Pas batholith, sensu
stricto, and presumed satellite intrusions of the De Pas batholith, which occur to the east of the
main batholith. The satellite intrusions are corre- lated with the batholith on the basis of lithology.
Two samples of De Pas batholith megacrystic granite, from the main part of the batholith in the
southern Crossroads domain, have emplacement ages of 1831 9 5 Ma James et al., 1996 and
1811 9 3 Ma Krogh, 1986, as determined by U – Pb dating of zircon. One of the presumed
satellite intrusions, consisting of strongly foliated megacrystic granite, is dated at 1823 9 5 Ma
James et al., 1996. From farther north in the batholith, Dunphy and Skulski 1996 have deter-
mined that a foliated De Pas batholith tonalite has an emplacement age of 1840 Ma on the basis
of preliminary U – Pb dating of zircon. In an attempt to obtain minimum ages of em-
placement for De Pas K-feldspar megacrystic granite, and to constrain the timing of deforma-
tion that overprints these rocks, a unit of isotropic to very weakly foliated, pink, biotite monzogran-
ite containing xenoliths of strongly foliated K- feldspar megacrystic granite was sampled. On the
basis of lithology and structure, the xenoliths are correlated
with foliated
De Pas
K-feldspar megacrystic granite. Field relations suggest that
the biotite monzogranite CR7 is late syn-tec- tonic with respect to the deformation in the in-
cluded megacrystic granite. Two fractions of concordant zircons from sample CR7 Fig. 13
yield an age of 1810 9 3 Ma, interpreted to repre- sent the crystallization age of the rock. This age is
within error of the youngest emplacement ages from the De Pas batholith. The strong foliation in
the batholith is inferred to have formed between 1810 and 1823 Ma.
5. Discussion
5
.
1
. Archean e6olution Field, geochronological and Nd-isotopic data
indicate that the Crossroads and Orma domains contain a significant component of Late Archean
granite – greenstone terrane crust. Supracrustal rocks in both domains are undated but are con-
strained to be older than 2704 Ma. They are intruded by tonalite to granite plutons in the
interval between ca. 2704 and 2620 Ma; these are inferred to be pre- to syn-tectonic with respect to
high-grade metamorphism and attendant defor- mation. Granitic intrusions correlated with the De
Pas batholith, and mafic dykes correlated with the 1817 – 1809 Ma dykes, are discordant to super-
posed folds of metamorphic leucosome in the supracrustal rocks in the Crossroads domain, sug-
gesting that these features are Archean. A possible interpretation of the data is that Archean rocks in
Crossroads and Orma domains have a similar Archean depositional, intrusive and tectonother-
mal evolution.
Fig. 12. U – Pb concordia diagram for sample CR6.
Fig. 13. U – Pb concordia diagram for sample CR7.
The McKenzie River domain lacks Archean supracrustal rocks and the ca. 2776 Ma Flat Point
gneiss is significantly older than intrusive units in the Crossroads and Orma domains. In contrast to
the isotopically juvenile intrusions in the Cross- roads and Orma domains, negative o Nd values,
at 2800 Ma, and T dm ages \ 3.0 Ga Kerr et al., 1994 from the Flat Point gneiss suggest that the
rocks incorporated a component of older crust. Archean rocks in the McKenzie River domain
were probably not part of the same granite – greenstone terrane that makes up the Crossroads
and Orma domains.
Parentage of Archean rocks that occur in the Core Zone of the southwestern SECP is uncertain.
Archean data from these rocks is non-unique and could be used to support radically different mod-
els. However, the fact that the entire exposed margin of the Superior craton shows evidence of a
significant rifting event, initiated between ca. 2.2 and 2.0 Ga, and reflected in the development of
Cycle one rocks in the southern Labrador Trough, strongly suggests that even if Archean
rocks in the Core Zone have a Superior craton affinity, they acted as independent crustal blocks
after ca. 2.0 Ga. Moreover, if the De Pas batholith is a subduction-related magmatic arc,
then Paleoproterozoic ocean basins must have separated distinct cratons or blocks consisting of
Archean crust prior to ca. 1.84 Ga.
5
.
2
. Paleoproterozoic e6olution The oldest Paleoproterozoic rocks in the south-
ern Crossroads domain Fig. 14 are ca. 1835 – 1810 Ma K-feldspar megacrystic granite, granite,
granodiorite and charnockite intrusions of the De Pas batholith. These granitoid rocks are variably
deformed and recrystallized demonstrating they predate and overlap with a tectonothermal event,
which persisted to at least 1775 Ma on the basis of U – Pb titanite data from other rocks the do-
main. Paleoproterozoic metamorphism in the Crossroads domain reached amphibolite facies,
but it did not result in the production of meta- morphic leucosome. Major- and trace-element
geochemistry of De Pas batholith rocks define calc-alkaline trends Van der Leeden et al., 1990;
Dunphy and Skulski, 1996, and are compatible with an interpretation for the batholith as a conti-
nental magmatic arc formed above an east-dip- ping subduction zone Martelain, 1989; Van der
Leeden et al., 1990; Dunphy and Skulski, 1996. In apparent contradiction to this interpretation,
Kerr et al. 1994 have noted that De Pas batholith rocks do not have all of the geochemical
signatures characteristic of modern subduction-re- lated continental magmatic arcs; they note, as one
example, that most samples show a strong enrich- ment in Zr. However, some of the geochemical
signatures, which are not characteristic of modern arcs, may in part be related to the nature of the
host Archean crust and to how much of the host was assimilated during emplacement of the De
Pas batholith. The batholith rocks are character- ised by negative o Nd values between − 3 and
−
7, calculated at 1830 Ma, and T dm ages between 2.24 and 2.64 Ga indicating that De Pas
magma was significantly contaminated by the sur- rounding Archean crust Kerr et al., 1994; Dun-
phy and
Skulski, 1996.
The geochemical
signatures of De Pas batholith rocks are inter- preted to reflect the mixing of juvenile, subduction
related magma and Archean crust of the Core Zone.
There is a need for additional field and isotopic data from the Orma domain, although existing
U – Pb geochronological data collected by Nunn et al. 1990 suggest that Archean rocks in the
domain have not been overprinted by a Pale- oproterozoic tectonothermal event. The analysed
minerals do not show any significant post- Archean Pb loss, nor do the samples contain new
Paleoproterozoic zircon or titanite. However we suggest that the domain probably contains Pale-
oproterozoic intrusions petrogenetically related to, and part of, the De Pas batholith. In particu-
lar, we provisionally correlate a unit of foliated, pyroxene-bearing K-feldspar megacrystic granite
mapped by Nunn, 1994 Nunn’s Unit 5, page 433 with the De Pas batholith. That these rocks
are foliated and metamorphosed; pyroxene is rimmed by hornblende and the rocks contain
garnet, suggests that at least locally the Orma domain contains Paleoproterozoic structures and
amphibolite – facies minerals.
Fig. 14. Summary diagram of U – Pb geochronological data Paleoproterozoic ages for the study area. Evidence for coeval metamorphism and deformation in McKenzie River and Crossroads domains after ca. 1810 Ma, and different magmatic histories
prior to that, suggests the domains were juxtaposed by that time. Included are U – Pb ages for De Pas batholith rocks from 1 James et al., 1996 2 Krogh, 1986 and 3 Dunphy and Skulski, 1996.
The proposed correlation in the preceding para- graph is significant because it implies that move-
ment along the George River shear zone, which postdated emplacement of the De Pas batholith is
probably not significant in the southern part of the Core Zone. This interpretation is consistent
with field and geochronological data indicating that deformation in the George River shear zone,
170 km north of our study area, was occurring but waning at 1825 Ma Dunphy and Skulski,
1996. Furthermore, the fact that the Crossroads and Orma domains have very similar Archean
geology suggests that in the study area, the George River shear zone does not have a signifi-
cant total-finite transcurrent displacement, and that the two domains have acted as an essentially
intact Archean block throughout their history. Paleoproterozoic ca. 1825 – 1775 Ma amphibo-
lite – facies metamorphism and deformation is more pervasive in areas contiguous with the De
Pas batholith i.e. in the Crossroads domain, consistent with models for metamorphism in a
magmatic arc setting. This may explain why the Orma domain, which is mainly lacking in De Pas
intrusive rocks, effectively escaped Paleoprotero- zoic tectonothermal effects. Absence of a late syn-
to post-De Pas batholith metamorphism in the Orma domain is also consistent with tectonic
models for the batholith which predict that it is exposed as a tilted, oblique section having deeper,
higher-grade rocks in the west and lower-grade rocks in the east Dunphy and Skulski, 1996.
The McKenzie River domain does not contain De Pas batholith granitic rocks, suggesting the
McKenzie River and Crossroads domains were not in their present configuration until after ca.
1810 Ma, the youngest emplacement age for De Pas granite in the region. An occurrence of ca.
1815 Ma tonalite indicates igneous activity in the McKenzie River domain at this time, although
the significance of this tonalite is uncertain. Leu- cosome from amphibolite – facies Lobstick group
metasedimentary rocks, dated at ca. 1805 Ma, indicates that the peak of metamorphism in the
McKenzie River domain falls within the range of metamorphic monazite and titanite ages from the
Crossroads domain. These data and the fact that deformation in the Lac Tudor shear zone was
attendant with amphibolite – facies metamorphism suggests that the McKenzie River and Crossroads
domains were linked by ca. 1805 Ma. A recrystal- lized tonalite dyke, which cross-cuts the metamor-
phic leucosome and strong foliation and is dated at 1802 + 9 − 14 Ma, indicates that metamor-
phism outlasted deformation in the McKenzie River domain. Ages of metamorphic monazite
and titanite from the Crossroads and McKenzie River domains this study are consistent with the
ages of metamorphic monazite 1793 9 5 and 1783 9 2 Ma and titanite 1774 9 5 and 1783 9
11 Ma in orthogneisses from the Kuujjuaq do- main Machado et al., 1989, 300 km north of the
study area, and support widespread metamor- phism at this time.
6. Tectonic model
